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Activity 13 : IMAGE Satellite Scaling

Introduction

Scientists need to consider size, mass, and cost when designing a satellite.
Although more data is returned by larger satellites, it isn't simply
a matter of doubling or tripling the dimensions of a satellite that lead
to a good design. When you double the size of a design, the volume increases
by 8 times and so does the mass and the cost of the satellite. The IMAGE
satellite is designed to be built for under $30 million, and have a mass
of 70 kilograms, but doubling its size causes the mass to increase to 1/2
ton, and its cost to rise to $240 million!

Objective

The students will calculate the volume of the satellite. Based on the results,
the students will determine patterns and relationships to see how change
affects the mass and the cost of the instruments.

Materials

Procedure

Read the students the introduction to the IMAGE Dimension Activity, or
discuss the key points with the students.

Allow students to complete questions 1,2, and 3. Discuss student responses
to these questions and the implications of increasing the size of the instruments
on the satellite. Refer to the Introduction as necessary.

Discuss how the change in the dimensions affects the mass. Allow students
time to complete activity 4. Discuss the results.

Discuss how the cost of the satellite is affected by increasing the dimensions
and how it is proportional to the mass. Allow students to complete question
5. Discuss the results.

Have students complete questions 6 and 7. Provide time for students to
give possible responses. Discuss why a scientist may want a bigger satellite
and the implications of scaling to the independent components.

Introduction to the IMAGE Dimension Activity

The IMAGE satellite must meet certain requirements. It must meet a two
year mission lifetime, and it must be launched on a Med-Lite rocket with
a payload mass limit of 100 kilograms. It must cost less than $50 million.

The scientists need to consider mass, cost, and volume when designing
a satellite. There are numerous instruments that need to be scaled, and
all of the factors mentioned above must be considered.

For instance, if a detector is chosen, the dimensions need to be investigated
to determine the final instrument cost. As in the activity, if a dimension
is doubled, the mass increases by 2 x 2 x 2, or a factor of 8. In turn,
the cost increases in proportion to the mass, which in this case is also
a factor of 8.

In addition, any change in the dimensions impacts on the other instruments
in the satellite, the labor needed to assemble it, salaries, and the launch
vehicle required for the extra mass and size. The IMAGE satellite will
require a rocket to launch it. If the satellite's size is quadrupled, it
would require the Space Shuttle to launch it. The difference in the launch
cost alone is $50 million for the rocket compared to $700 million for the
Space Shuttle.

The major constraint on the IMAGE project is cost. The IMAGE satellite
must be built, launched, and operated for 1 year at a total cost less than
or equal to $60 million. Nearly all of the cost of any mission is in the
salaries of the scientists and the engineers needed to manufacture the
satellite and the rocket.

In response to the question of why a scientist would want a bigger satellite,
it is purely based on the amount of information that is being sought. The
IMAGE satellite is designed to provide 2-D images of various phenomena
in space. An increase in the dimensions will cause an increase in the size
due to the need to acquire more data. Doubling the array size, in turn,
quadruples the amount of data it can capture and send down to the scientists
on the ground.

However, the question of size is more involved than this. The instruments
of the IMAGE satellite, which have been the major focus, are not the only
things to consider in satellite design. In terms of scaling, when the size
of the instruments double, it does not necessarily mean that the size of
the satellite will double. Compared to the overall size of the satellite,
the instruments are only a small part of the total size compared to other
satellite systems and hardware. The components of the satellite can be
scaled independently. Doubling the size of one instrument may not necessarily
double the size of the electrical power required to operate it, and so,
double the size of the power supply needed to provide the electricity.

Conclusions

When designing a spacecraft, it is important to know how the cost and the
capabilities of the satellite will scale as you change its dimensions.

Related Web Resources

Visit the IMAGE Satellite
Picture Gallery to view
images of the spacecraft.